This paper analyses the impact of the new Basel Capital Accords (Basel II and Basel III) on the bank’s capital requirements in a portfolio of Small and Medium-sized Enterprises (SMEs) when the internal ratings-based (IRB) approach is used. To do this, the study uses a large database of Spanish firms and covers the period from 2005 to 2009. We also examine the effect on the credit risk premium charged by banks of the guarantee offered by a Loan Guarantee Association (LGA) to a SME; and whether this foreseeable decrease in the interest rates applicable to the SME is compensated by the cost of this guarantee.